Fractures of the bones in the human body are often fixed with implants that include plates and bone fasteners, such as pins and screws. The holes in these plates are oriented along an axis that is perpendicular to the surface of the plate, since this avoids "walking" of the drill as the hole is drilled in the plate and results in a cleaner hole. Furthermore, since the majority of screws are commonly inserted in an orientation that is perpendicular to the surface of the plate, this perpendicular orientation of the hole in most cases is the optimal design.
Occasionally, because of the local configuration of the fracture, it may be required to insert a screw at an angle that is not perpendicular to the surface of the plate, namely, an acute angle less than 90.degree.. Since the typical screw hole is, in general, slightly oversized, the surgeon is able to angle the screw during insertion between opposite sides of the hole. Because the holes are placed perpendicular to the plate, the possible range of insertion angles is symmetrical from side to side. However, since most screws are placed in the shaft, or central region, of bone, this ability to angle the screw in opposite directions is often a useful feature.
There are some situations, however, during the fixation of fractures in which the local anatomy and/or geometry will only permit safe insertion of a screw within a predetermined limited range of screw insertion angles which is not symmetrical relative to an axis perpendicular to the plate surface. For instance, such a situation can occur during the application of a plate to the distal end of the radius, where certain directions of screw insertion to one side would result in penetration of the screw across the joint and into the articular surface, resulting in severe, progressive posttraumatic arthritis. Currently, there are no designs available that limit the possible range of insertion angles to a predefined range of values that is not symmetrical with respect to an axis perpendicular to the surface of the plate.
If a larger hole is provided in the plate to accommodate large oblique angles of insertion of the screw, this is not optimal for two reasons. First, this would allow the oblique angle of insertion to occur to either side of the plate which may not be desirable, such as in the case referred to above. Secondly, simply increasing the hole size in the plate results in the unwanted side effect of excessive lateral translation (or slop) of the screw in the screw hole. Rather, the optimal design should not only permit a predefined asymmetric range of insertion angles, but additionally provide a hole in the plate that is as small as possible, in order to limit translational movement of the screw in the plate.
The preceding discussion is equally applicable to the use of pins instead of screws through the plate hole.